György BUDA¹, Géza NAGY² & Elemér PÁL-MOLNÁR^3,4
1Department of Mineralogy, Eötvös Loránd University, H-1117 Budapest, Pázmány P. Street 1/C, Hungary
e-mail: buda@ludens.elte.hu
²Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi Street 45, Hungary; e-mail: gnagy@geochem.hu
³Department of Mineralogy, Geochemistry and Petrology, University of Szeged, H-6701 Szeged, P.O. Box 651, Hungary
4MTA-ELTE Volcanology Research Group, H-1117 Budapest, Pázmány P. Street 1/C, Hungary
e-mai: palm@geo.u-szeged.hu

ALLANITE AND MONAZITE OCCURRENCES IN VARISCAN GRANITOIDS OF TISZA MEGA-UNIT (SOUTH HUNGARY)

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Two main Variscan granitoid plutons are in the Tisza Mega-Unit of south Hungary. The older one (~340 Ma) belongs to the magnesio-potassic, met- to peraluminous calc-alkaline monzonitic series with abundant mafic enclaves in Mórágy Subunit. In the whole intrusive body, allanite-(Ce) crystallized along with apatite and titanite as accessory minerals due to high Ca activity. The high concentration of Ca and low Al in the melts favoured the crystallization of allanite. The chondrite-normalized REE pattern of the host rock is controlled by the REE-rich allanite-(Ce). High oxidation of the unaltered allanite indicates crystallization under high oxygen fugacities from high-temperature I-type granitoid magma. Allanite crystallization was continuous over the entire magma solidification, as revealed by the abundant presence of rock-forming mineral inclusions. Volatile-rich post-magmatic hydrothermal solutions altered the allanite-(Ce) in different ways depending on the pH of solutions. Alkali-rich solutions leached Si and Al, whereas acidic fluids dissolved and transported the REE, depositing them on the rims of crystals or in nearby cracks as a form of fluorcarbonates. Some zoned crystals preserved the original magmatic REE zonation after leaching of Al and Si. The younger plutons (300–325 Ma) occur in the southeast part of the Tisza Mega-Unit in the Battonya Subunit. They consist of peraluminous, monazite-rich, high Al, low Ca, two-mica and muscovite granodiorite-granite with mafic enclaves. The muscovite-rich granitoids are S-type and belong to the monazite-type calc-alkaline granodioritic series with some metaluminous amphibole-bearing enclaves.